The Runner and the Wizard — Dave Duncan

Young Ivor dreams of being a swordsman like his nine older brothers, but until he can grow a beard he's limited to being a runner, carrying messages for their lord, Thane Carrak. That's usually boring, but this time Carrak has sent him on a long journey to summon the mysterious Rorie of Ytter. Rorie is reputed to be a wizard-or an outlaw, or maybe a saint-but the truth is far stranger, and Ivor suddenly finds himself caught up in a twisted magical intrigue that threatens Thane Carrak and could leave Ivor himself very dead.